Conventional devices for measuring radiation damage caused by neutrons of varying energies, such as neutron dose rate meters, also known as rem meters, typically provide a response as a complex function of energy, corresponding to the so-called “conversion factors” theoretically calculated to represent the neutron radiation damage per unit of neutron fluence at a given neutron energy. The conversion factors are typically those recommended by ICRP (International Commission on Radiation Protection) publications, and their use is regulated by governing authorities such as the US Department of Energy.
The correspondence of conventional neutron detector responses to the set of conversion factors must be provided by the rem meter. Such detectors include neutron sensor devices having a good efficiency of detecting low energy “thermalized” neutrons, with those devices being contained in the “neutron moderator” envelopes. The moderators convert incident fluence of neutrons at any energy to the fluence of thermalized neutrons at the region around the sensor, such that the reaction rates correspond to the pre-calculated theoretical dose conversion factors. In this manner the detector response may be calibrated to be in rem units in a range of neutron energies suitable for the typical neutron radiation environment to be monitored.
Early neutron rem meters were designed to operate around nuclear power plants and their energy response was correspondingly limited at about 10 MeV. The moderators typically used polyethylene envelopes, with extra neutron absorbers inside to make the response functions more leveled.
However, neutron dose rate measurements at particle accelerators typically require neutron rem meters capable of detecting neutrons with energies above 10 MeV. One proposed method for solving this problem includes using a layer of a high-Z material inside of a poly moderator, such as the WENDI-type (Wide Energy Neutron Detection Instrument-type) moderator. The response functions of the WENDI-type neutron moderators generally exhibit rise of sensitivity to higher energy neutrons due to the (n, Xn) neutron multiplication reactions in the heavy nucleus, at energies above approximately 20 MeV. Generally, such type of moderators under-respond in the range of thermal neutron energies (1-100 meV) and in the range of 10-30 MeV where the contribution of the multiplication reactions is still relatively small, but they over-respond at higher neutron energies such as above 1 GeV.
Accordingly, one purpose of the present invention is to provide a higher sensitivity neutron moderator for use in rem meters with the response function better corresponding to the theoretical conversion factors. Another purpose of the present invention is to generally improve the energy response of such detectors.
Furthermore, reliable measurements of neutron dose rates at high energy electron accelerators and photon therapy facilities, in the conditions of high gamma radiation (γ-radiation), are needed in many instances including, for example, evaluations of damage to materials and electronics components, and monitoring of neutron doses to patients. Conventional devices are not capable of measuring neutron dose rates in the presence of high gamma radiation.
Accordingly, a method of detecting neutron doses in strong γ-radiation environments would be highly beneficial.